1. Field of the Invention
The present invention relates to metal vapor discharge lamps such as a high pressure mercury vapor discharge lamp in which mercury and a rare gas are filled; a high pressure sodium vapor discharge lamp in which mercury, a rare gas and sodium are filled; and a metal halide vapor discharge lamp in which mercury, a rare gas and a metal halide are filled. More particularly, it relates to metal vapor discharge lamps which is started by an operation of a heat sensitive switch means such as a bimetallic switch and has an advantage for preventing a ballast from overheating in a fused bond of contacts of the bimetallic switch.
2. Description of the Prior Arts
A metal halide lamp will be described as one embodiment.
The metal halide lamp contains mercury, a rare gas and a metal halide as a discharge sustaining fill and has superior efficiency and color rendering property to those of a high pressure mercury vapor discharge lamp.
FIG. 1 shows a structure of a metal halide discharge lamp which can be operated on a conventional ballast for a high pressure mercury vapor lamp.
An outer bulb made of a light transmissable hard glass (1) covers an arc tube (2) made of a transparent quartz glass in which a rare gas such as neon-argon or neon-krypton and a desired quantity of mercury and a metal halide are filled. Main electrodes (3a), (3b) are placed at both ends of the arc tube (2) to face each other and are connected through metal foils (4a), (4b) made of molybdenum to leads (5a), (5b). The lead (5a) is connected through a filament (7) for operating a bimetal (6) as a heat sensitive switch means to a stem lead (8a). The other lead (5b) is connected through a current feeding wire (9) made of tungsten etc. to the stem lead (8b).
An auxiliary electrode (10) is placed to face the adjacent main electrode (3a) and is connected through a metal foil (11) to a lead (12).
The lead (12) is connected to a fixed contact (14) placed on a glass bead (13) for holding the bimetal (6) and is further connected through a movable contact (15) placed at one end of the bimetal (6) and the bimetal (6) and a fixed terminal (16) to the stem lead (8b). An outer coating (17) for increasing the densities of the metal halides filled in the arc tube (2) at the operation is placed at the end of the arc tube (2).
A Zr-Al type getter is placed in the outer bulb (1) and a Ne-N.sub.2 mixed gas having a desired pressure is filled for preventing Ne gas from permeating through the arc tube (2). A stem (19) and a base (20) are equipped.
When the metal halide discharge lamp having said structure is operated by connecting a power source through a ballast for a high pressure mercury vapor discharge lamp, an auxiliary discharge is initiated between the main electrode (3a) and the auxiliary electrode (10) and then, the bimetallic switch (6) is operated by the heat generated from the arc tube and the filament (7) after the continuation of the auxiliary discharge for a predetermined time whereby the connection between the fixed contact (14) and the movable contact (15) is disconnected. As in the arc tube (2), the ionized gas is formed by the auxiliary discharge, the main discharge is initiated easily when a surge voltage induced by the ballast is applied between the main electrodes (3a), (3b). But, the phenomenon for the discharge between the fixed contact (14) and the movable contact (15) and between the fixed contact and the bimetal (6) is found. Especially, after prolonged operation of the lamp, if the main discharge between the main electrodes (3a), (3b) is not stabilized after the disconnection of the bimetallic switch, the discharge between both of the contacts (14), (15) is repeated and the contacts are finally fuse-bonded to prevent the normal discharge between the main electrodes whereby a large current substantially equal to a short-circuit current of the ballast continuously flows between the main electrode (3a) and the auxiliary electrode (10) disadvantageously to cause overheating the ballast.